RF-based fiber-to-the-home (FTTH) systems have been developed for cable television operators who want an alternative to the standards-based passive optical networks (PON). One benefit of such systems is that they are transparent to the RF signals they carry, and this allows an operator to continue to use the same customer premise equipment (CPE) that is used on their hybrid fiber/coax (HFC) networks. This equipment includes set-top boxes, DOCSIS cable modems, and DOCSIS VoIP modems.
In RF-based systems, the network interface unit (NIU) placed at the customer location contains a laser for upstream optical transmission. The laser utilizes burst mode transmission in the upstream direction and is inactive until an RF signal is generated by one of the CPE devices in the home. When that signal (which may be modified before reaching the laser) reaches the upstream laser it activates the laser by directly modulating its optical output. When the RF burst ends, the laser returns to its inactive state. Because of the protocol used by the cable modems, only one modem is active at a given time on a given RF upstream channel.
With only one modem active at a time, only one NIU on the relevant portion of the fiber network will be transmitting at a given time. However, when there is more than one upstream channel for a modem to use it is possible to have more than one NIU transmitting at a given time. For example, one NIU may be transmitting a signal from a cable modem operating on a first channel, while another NIU is transmitting a signal from a cable modem operating on a second channel. In this case there will be an optical collision at the upstream receiver. A similar problem may occur when a cable modem from one location is transmitting, and a set-top box (STB) from another location is also transmitting. Since the STB and cable modems operate independently, there is no synchronization and therefore the transmission timings of the two systems may overlap.
The systems (whether set-top boxes or cable modems on one or more upstream channels) operate on different RF frequencies; therefore, if the operating wavelengths of the two lasers do not significantly overlap, the result of the optical collision will simply be a degradation in link performance. When the laser wavelengths do significantly overlap a mixing occurs in the optical receiver which results in the generation of an RF signal who frequency is at the difference between the operating wavelengths of the two lasers. This signal can be sufficiently large that it overdrives the input of all the upstream systems attached to that receiver and makes a channel non-functional for the duration of that burst. Similar problems will occur in any future burst if two or more NIUs are simultaneously active and their wavelengths significantly overlap. Because of the protocols associated with the CPE devices, this could knock the devices off of the network and cause them to re-initialize on the network.
One of the difficulties in resolving this problem is that the operating wavelength of a laser is dependent on its temperature, and NIUs are often used in an outside plant environment where temperatures can range from −40 to +65° C. In addition, one NIU may be in the sun while another is in the shade, and this causes a temperature differential between the two units. The operating wavelength of a laser changes by about 0.1 to 0.5 nm/° C. depending on laser design. Therefore, while a system may nominally include lasers transmitting at different wavelengths, these wavelengths may change with changing temperature over the course of a day. The wavelength of one NIU may thus walk through the wavelengths of other NIUs on the network or may reach a temperature that parks it on the same wavelength as that of another NIU.
One way of addressing this problem is to use lasers in NIU's connected to a given receiver that have sufficiently spaced wavelengths so that, over the expected temperature range, they will never overlap. In other words, each NIU in a group served by a particular receiver would require a different wavelength. However, this approach is not entirely satisfactory because it increases the system cost and complexity. Under this approach, lasers must be sorted into groups that meet the requirements of a particular operating window in a system, much like coarse wavelength division multiplexing (CWDM). In addition, the solution requires an operator to stock complete sets of NIUs (each having a unique wavelength) which could be as many as 32 per set. This may become difficult to manage when deploying and subsequently maintaining a system. It also takes up spectrum on the fiber that could be used to support other services such as business overlays.
It would therefore be desirable to provide a system that reduces the problem of interference between lasers transmitting upstream to a receiver which system does not require the use of lasers operating at widely spaced wavelengths.